Electromechanical rotary actuators are well known and are used in a variety of industrial and consumer applications. They are particularly useful in the field of optical scanning, where an optical element is attached to an actuator output shaft, which is then rotated back and forth in an oscillating manor.
For example, it is common to attach a mirror to the output shaft of a rotary actuator in order to create an optical scanning system. In this application, the actuator/mirror combination can redirect a beam of light through a range of angles, or redirect the field of view of a camera so that it can observe a variety of targets.
Other optical elements can be attached to the output shaft as well. For example, a prism or an optical filter can be attached to the shaft and the rotation of the actuator shaft can vary the angle of the prism or filter. If a dielectric filter is used, changing the filter's angle-of-incidence will shift the band-pass wavelength characteristics higher or lower, thus allowing the optical system to be tuned to a particular wavelength. Alternatively, the prism or filter can be rotated completely into and out of the beam path, thus allowing selective filtering of the beam.
Yet another application is to attach an arm to the actuator output shaft, with the arm being made of opaque material such as blackened metal. The rotation of the actuator shaft rotates the arm into and out of the beam path, thus providing a shuttering action.
Many well known rotary actuators provide only two discreet rotation angles, and the purpose of the actuator is to vary the output shaft between these two angles in a kind of digital, on-off fashion. These actuators are also usually accompanied by a type of mechanical shock effect (vibration), where the rotating inertial load must suddenly come to a stop at the end of angular travel. This mechanical shock is highly undesirable for optical applications, because this shock can be coupled to other optical elements, creating disturbances as well as acoustic noise.
In addition to shock-free actuation, optical scanning related applications also desire that the accessible range of rotation angles be virtually infinite, as well as being controllable and repeatable, in an analog fashion. Sometimes a rotation angle of 5 degrees might be needed, and other times a rotation angle of 10 degrees might be needed. Still other times, some intermediate angle might be needed, for example 6.54 degrees.
With the desirable range of output angles virtually infinite, there is a requirement that there be some method of controlling the actuator output angle, based on an external signal. To this end, two methods exist—open-loop control and closed-loop control.
When open-loop control is used, the actuator generally must have some spring-like return mechanism, such that when no current is applied to the actuator, the spring-like mechanism will return the shaft to a nominal angle. Then, when the amount of input current that is applied to the actuator is varied, this will vary the amount of torque produced by the actuator, and thus vary the amount of torque applied to the spring, which then will control the output angle of the actuator. In this way, there is a direct relationship between the output angle produced by the actuator and the input current applied to the actuator. However, the degree of linearity of the open-loop control depends strongly on the torque-versus-angle characteristics of the actuator, and also the torque-versus-angle characteristics of the spring-like return mechanism. Hysteresis effects within the materials or construction may also degrade the output angle versus input current relationship, and thus degrade repeatability. And finally, the speed of open-loop control depends on the amount of overshoot that is acceptable. If higher speeds are required, normally more sophisticated control methods are needed to artificially add damping to the system in order to control overshoots.
When closed-loop control is used, the actuator must incorporate an angular position sensor, which is generally externally attached. A servo system then applies current to the actuator to move the shaft in a direction to minimize the difference between the external commanded angle and the actuator output angle sensed by the angular position sensor. Closed-loop control can provide much greater speed, linearity and repeatability, but is of course more complex and more expensive, due to the required angular position sensor and servo control electronics.
Whether open-loop control or closed-loop control is used, it is desirable within the field of optical scanning, that the performance of the actuator be predictable when external current is removed—such as return the output shaft to a nominal, central angular position. In many well known actuators, this return-to-center action is provided by a metal spring, which can be a coil spring, leaf spring or torsion rod. In yet other known actuators, the magnetic construction or additional magnets are used to return the actuator to the center.
As it pertains the return-to-center mechanism, while metal springs can provide a linear return-force-versus-angle characteristic over a range of angles, there is a finite angular range over which they can work as desired, which is normally 25 degrees or less. Exceeding the designed range of angles will result in greatly reduced lifetime or even instantaneous breakage of the spring. And while magnetic construction techniques or additional magnets can provide a return-to-center action that does not fatigue or break, the return-force-versus-angle characteristic is generally not linear and in fact, can be highly non-linear.
Within the field of optical scanning and also within other fields, it can be desirable for the actuator to provide as wide an angular output range as possible. When a mirror is attached to the output shaft, a wider angle from the actuator provides a wider scan angle. When an opaque element is attached to the actuator, a wider angle from the actuator provides a greater degree of shuttering. However, well known commercially available actuators have not been found which provide an angular range greater than around +/−25 mechanical degrees along with analog control capability.
There is a need for an electromechanical rotary actuator that provides wide angle capability, and that can provide a linear current-versus-angle characteristic. Yet further, there is a need for such an actuator to also provide a self-damping characteristic to improve the speed when used with open-loop control.